A standard drill chuck has a chuck body that is mounted at the front end of a drill spindle and rotated about its center axis. This body carries a tightening sleeve formed with a plurality of inclined guides carrying respective jaws having teeth meshing with a screwthread of the chuck body. It is also known to provide the guides on the chuck body and the screwthread on the sleeve for the same effect. The edge of the tightening sleeve may also be provided with teeth that can mesh with teeth on a chuck key fitted to a pilot hole in the chuck body to bring extra force to bear. Rotation of the sleeve in one direction moves the jaws axially forward and radially toward one another and rotation in the opposite direction moves them axially backward and radially apart. Thus it is possible by rotation of this sleeve to clamp a tool (or workpiece) in the jaws and by opposite rotation to release this tool (or workpiece).
To chuck or dechuck a tool it is standard for the user to grasp the tightening sleeve and then actuate the drill's trigger, with the rotation-direction switch set appropriately according to whether the chuck is to be tightened or loosened. If the chuck is also of the key type the key is inserted to break the chuck free at the start of a dechucking operation or to give the final tightening at the end of a chucking operation.
It takes a fairly skilled person who is quite familiar with the drill in question to safely and skillfully perform such a tightening or loosening operation by hand with a motor assist. The potential for injury is not small, and the difficulty of accurately bringing just the right amount of torque to bear by hand is considerable.